This invention relates to magnetoresistive heads using the magnetoresistive effect by which information is reproduced from a magnetic recording medium, and magnetic heads by which recording and reproduction are carried out.
A magnetic head in a hard disk drive (HDD) provides the recording head which records information on a magnetic recording medium (hard disk) as magnetized signals and a reproducing head which reads the information recorded on the magnetic recording medium. The reproducing head is a magnetoresistive stack which comprises multiple magnetic thin films and non-magnetic thin films, and is referred to as a magnetoresistive (MR) head because signals are read out by use of the magnetoresistive effect. There are several types of stack structures for MR heads and they are classified, according to the principle of magnetoresistance used therein, into AMR (Anisotropy Magnetoresistive) heads, GMR (Giant Magnetoresistive) heads, CPP-GMR (Current Perpendicular Plane-GMR) heads, and TMR (Tunnel Magnetoresistive) heads. The magnetic head utilizes the AMR effect, GMR effect, CPP-GMR effect, and TMR effect, respectively, to read magnetic fields input on the magnetic recording medium and which, when they enter the magnetic head, change the voltage of electric current in the magnetic head.
When one magnetic layer in the magnetoresistive stack of an MR head is a free layer whose magnetic orientation is rotated upon receiving an external magnetic field from a magnetic recording medium, and that free layer has multiple magnetic domains, movement of magnetic domain walls occurs, causing noise. To suppress various noises such as Barkhausen noise, or to control asymmetry of the output signal, it is important to make the free layer have a single magnetic domain, widthwise, in the track.
In a method for controlling magnetic domains to make a free layer have a single magnetic domain, for example, as shown in Japanese Patent Laid-Open No. 125311/1993 and FIG. 9, there is a magnetoresistive stack which comprises an anti-ferromagnetic film 5, a soft magnetic film 4 (referred to as a pinned layer), a non-magnetic layer 3, a free layer 2 and a cap layer 1. A laminated stack having a seed film 9 and a ferromagnetic film 8 is disposed with a lead 6 thereon through another seed film 7 so that it is adjacent to each layer of the magnetoresistive stack. This method, which uses a magnetic field generated from magnetic film 8 widthwise in the track, is called hard bias.
In another representative method for controlling magnetic domains, as shown in U.S. Pat. No. 4,663,685 and FIG. 10, there is a magnetoresistive stack which comprises an anti-ferromagnetic film 5, soft magnetic film 4 (referred to as a pinned layer), a non-magnetic layer 3, a free layer 2 and a cap layer. In a method called patterned exchange, a lead layer 10 is formed on both ends of free layer 1 in such a manner that a seed film 11 is sandwiched between lead layer 10 and an anti-ferromagnetic film 12, and exchange coupling between anti-ferromagnetic film 12 and free layer 5 takes place.
Further, as proposed, for example, by Japanese Patent Laid-Open No. 282618/1997, Japanese Patent Laid-Open No. 53716/1999 or U.S. Pat. No. 5,739,990 is a system in which, to realize high reproducing output, a dead zone area, not to be used for reading, is created at the end portions of the free layer in the track in a widthwise direction by making the distance between the leads smaller than the track width of the free layer, since the magnetic orientation of the free layer is made difficult to rotate by the intensity of the magnetic fields generated by the lamination. Furthermore, as shown in Japanese Patent Laid-Open No. 203634/1999, there is a method in which an anti-ferromagnetic film of uniform thickness is built up on the entire surface of the free layer.
Moreover, as shown in Japanese Patent Laid-Open No. 2001-84527, also proposed is a system in which a magnetic domain control film comprises a high coercivity film and a lamination of a ferromagnetic film or an anti-ferromagnetic film or both.
However, in a magnetic domain control system as shown in FIG. 9, in which lamination layers having ferromagnetic film are disposed at both sides of the free layer, the dead zone area is increased when the ferromagnetic film is made thicker to secure stability of the element. To reduce the dead zone area, magnetic domain control force may be weakened by a method in which the thickness of the ferromagnetic film is simply reduced. In such case, however, drawbacks occur, such as Barkhausen noise and waveform instability, or increase of output asymmetry. It therefore becomes necessary to have magnetic domain control force so as not to generate those problems.
In addition to the problems relating to the dead zone area, it becomes a significant problem that the ferromagnetic film is overlaid on an upper surface of the element. The overlaid portion of the magnetic film generates a magnetic field in the opposite direction to the magnetic domain control magnetic field, and in the magnetic domain controlling magnetic field distribution across the track width (Twr) direction of free layer 2 shown in FIG. 12, rippling occurs in the vicinity of a portion in which magnetic film 8 is disposed, at end portions of free layer 2 in the track width (Twr) direction. Together with this rippling, the magnetic field which is applied to the end portion areas of free layer 2 in the track width direction is decreased. As a result, instability of the element is increased. In addition, to suppress the instability, magnetic film 8 must be made thicker than necessary, and, as a result, a magnetic field stronger than necessary is applied over the entire free layer 2 so that sensitivity is damaged. It is difficult to eliminate the overlay of magnetic film 8 completely due to the process employed. Thus it is necessary to use ingenuity with regard to the magnetic domain control structure.
In contrast, in a magnetic domain control method in which the anti-ferromagnetic film is disposed as shown in FIG. 10, coupling a magnetic field affects only the portion at which the anti-ferromagnetic film 12 is in contact with the free layer 2; therefore, the dead zone problem does not occur, and this method is also advantageous in making narrower tracks. Further, there is no effect of the overlay of the magnetic film on the element. However, the exchange coupling magnetic field of anti-ferromagnetic film 12 and free layer 2 is smaller as compared with a case in which magnetic film 8 is used.
As shown in FIG. 12, it is preferred that a magnetic field does not exist in a magnetic sensing region, but the magnetic field intensity is extremely small compared with the magnetic field of the end portions in the track width (Twr) direction of free layer 2 in a case where magnetic film 8 is used. Further, when an apparatus such as HDD (hard disk drive)is in use, the exchange magnetic field of anti-ferromagnetic film 12 and the end portion of free layer 2 deteriorates due to heat generated by the magnetoresistive element, and the magnetic domain controlling magnetic field is further decreased. Therefore, the problem arises that the end portion area of the free layer 2, which should be fixed, has sensitivity, and read drift occurs so that the record data of an adjacent track is read and the error rate is increased. Further, there a process problem that, because the width Tw of the magnetic sensing region increases as the effective track width is broadened, the geometric track width Twr must be made smaller.
In addition, the method disclosed in Japanese Patent Laid-Open No. 282618/1997, Japanese Patent Laid-Open No. 53716/1999 or U.S. Pat. No. 5,739,990 reduces the effect on reproducing output due to the dead zone problem, but due to the weakness of the magnetic domain controlling force which is applied to the end portions widthwise in the track of the free layer disposed under the lead, read drift becomes a significant problem: side read and crosstalk deteriorate, and the effective track width is broadened. Further, in regard to a magnetic head shown in Japanese Patent Laid-Open No. 203634/1999, there is a concern that, since the present head has a very narrow track width, fixing the entire surface of the free layer to be a uniform magnetic field results in lowered sensitivity, and adversely results in the absence of a magnetic domain controlling magnetic field at the end portions of the free layer, widthwise in the track where magnetic domain control is particularly necessary.
Moreover, a magnetic head shown in Japanese Patent Laid-Open No. 84527/2001 does not have enough coupling force since the anti-ferromagnetic film and the high coercivity film are coupled directly or through the ferromagnetic film.